The present invention relates to a metal mold for glass forming wherein molten glass is placed into a metal mold and then molded into a glass product. More particularly, the present invention relates to no-swabbing metal mold having excellent high-temperature strength and releasability.
Metal molds for glass forming are generally made of cast iron or heat-resisting steel. In the metal molds made of these materials, when molten glass of a high temperature is placed into the metal mold and comes into contact with the mold surface, unfavorable phenomena occur such as deposition of an oxide on the surface of the glass, deterioration of the releasability of the glass from the mold due to high adhesion between the mold surface and the glass, and formation of fine irregularities, called "surface toughening," on the surface of the glass.
For this reason, in the prior art, it was common practice to apply a release agent composed mainly of graphite or sulfur onto the surface of a metal mold for glass. This oil-based release agent, however, should be manually applied at regular intervals, i.e., several times per hour. Further, it burns to deteriorate working environment. Furthermore, deposition of a graphite powder onto the surface of the glass product is unavoidable, resulting in deteriorated quality of the glass product. This has led to a strong desire for no-swabbing mold not requiring the use of any swab.
In order to solve these problems, the formation of a sprayed coating of tungsten or a deposited coating of TiN or the like on the surface of a base material, such as cast iron or heat-resisting steel, has been proposed in the art (Japanese Patent Laid-Open Nos. 56339/1982 and 156020/1988). In these proposals, a metallic layer for accelerating the release of the glass from the metal mold is separately applied after the production of the metal mold, and the production facilities, provision of additional production steps, and necessity of again forming the mold due to damage to the surface thereof increase the mold cost. In addition, the releasability is not satisfactory yet, thus making it impossible to realize no-swabbing mold which is free from maintenance with respect to the application of any lubricating/swab.
The present inventors have filed an application of an invention relating to a material, for high-temperature use, having excellent resistance to oxidation at high temperatures (Japanese Patent Laid-Open No. 73034/1989). This alloy material has excellent resistance to oxidation in an oxidizing atmosphere at high temperatures. However, when it comes into contact with glass having a high temperature above 1000.degree. C. or when it is used as a glass mold, which undergoes thermal shock during glass forming, an alumina film as an oxide film is partially peeled off, rendering the releasability unsatisfactory.
Thereafter, the present inventors filed an application of an invention relating to an alloy for no-swabbing glass mold which has a stable alumina film on the mold surface, a high coefficient of thermal conductivity, high hardness at high temperatures, and excellent corrosion resistance (Japanese Patent Laid-Open No. 10079/1994). Since, however, the structure of this alloy is composed mainly of a ferrite phase, the metal mold, due to lack of hardness at high temperatures, often causes abrasion loss and surface roughening by corrosion in the course of continuous operation for a long period of time. Further, heat treatment for forming an aluminum oxide film gives rise to the formation of a small amount of Cu oxide which deteriorates the wettability of the metal mold by the glass, often resulting in wrinkle defects on the surface of products at the time of glass forming. This requires the development of a further improved mold.
Furthermore, the present inventors succeeded in developing the so-called "no-swabbing molds" not requiring the use of any swab and filed applications of inventions relating to glass molds, having releasability enhanced by forming a very stable strong alumina film on the surface of the glass molds (Japanese Patent Laid-Open Nos. 158269/1994 and 279944/1994). The use of these alloy molds having excellent releasability without the application of any oil, however, caused galling in the contact area between parts of the mold, making it impossible to continue smooth molding operation and production of glass products with a good dimensional accuracy after occurrence of the galling. Consequently, the service life of these molds was equal to or shorter than that of the conventional metal molds.
The reason why galling occurs is as follows. In the conventional glass mold using an oil-based swab, the swab coated onto the surface of the mold flows out on the contact area between parts of the mold and consequently serves as a lubricating oil, thus preventing galling. On the other hand, in the case of the alloy mold, having excellent releasability of glass from the mold, disclosed in Japanese Patent Laid-Open Nos. 10079/1994, 158269/1994, and 279944/1994, since no oil-based swab is used, no lubricating component is fed to the contact area between parts of the mold, leading to galling which in turn shortens the service life of the mold.
The metal mold for glass forming is classified into a blank mold and a blow mold. Also in a mold of the conventional alloy, in the case of a blow mold, no swab is, in some cases, applied in order to avoid the deposition of a swab on the surface of a glass product. This causes galling in the contact area between the parts of the mold, resulting in shortened service life of the mold. The present inventors have made research and development also on a glass mold, with a stable aluminum oxide film formed on the mold surface and a novel method for lubricating the contact area between parts of a mold in a blow mold for glass wherein no swab is applied to the contact area between the mold and the glass.